This invention relates to a method and apparatus for controlling a phase-locked loop (PLL). In particular, it relates to a method and apparatus for controlling a phase-locked loop for a cellular telephone.
Cellular telephones typically utilize two mixers for receiving an RF signal. A first mixer, or RF mixer, converts the incoming RF signal to an IF frequency. After passing through the IF filter circuit, the output frequency is passed through a second, or IF mixer. This IF mixer converts the IF frequency signal to a base band signal. The base band signal is then converted to a digital signal which is fed to an input of a digital signal processor for further processing. In the receive mode, the IF mixer is held at a constant frequency but the RF mixer is varied in frequency in order to receive one of the multiple channels in the RF band in which the cellular telephone operates. Each mixer has its own local oscillator, which in turn are part of separate phase-locked loop circuits
TDMA (time division multiple access) cellular telephone systems, such as those utilized in the GSM standard, operate in a half duplex mode. This allows the local oscillator for both the RF and IF mixers in the receive circuit to be reused in the transmit circuit. The frequencies of operation for both the IF and RF mixer local oscillators are changed for the transmit mode. In order to reduce spurious frequencies generated in the transmit mode, it has been found necessary to change both the RF mixer local oscillators and IF mixer local oscillator frequencies with a particular set of RF and IF frequencies chosen for each channel on which the cellular telephone transmits, which has been specifically chosen to minimize such spurious transmissions.
The phase-locked loop circuits which generate the two local oscillator frequencies may both be on a single integrated circuit, for example. The operation of the two local oscillators is controlled by a digital signal generated by the digital signal processor of the cellular telephone via a serial interface. The serial interface transmits a 24 bit word to control the frequency of each of the local oscillators. Thus, to change the frequencies of the two local oscillators, requires a transmission of two 24 bit words. The transmission of the two 24 bit words is required when the cellular telephone enters the transmit mode. While in the transmit mode, the frequency of operation of a particular cellular telephone may be changed from time slot to time slot according to the-needs of the cellular telephone cell in which the telephone is operated. Each time the frequency on which the cellular telephone is transmitting is changed, the two 24 bit words must be resent.
Each time the cellular telephone enters the transmit mode the digital signal processor has many tasks to perform. The time necessary to transmit these two 24 bit words creates a difficult timing environment for the programmer of the digital signal processor. In addition, the memory required for this procedure adds to the cost of the cellular telephone.
It is a general object of the present invention to produce a control circuit for a phase-locked loop circuit.
Another object of the present invention is to produce control circuit for a phase-locked loop circuit for a cellular telephone.
A further object of the invention is to control the operation of the RF mixer local oscillator and IF mixer local oscillator utilizing a single control word having fewer bits than required to control the two local oscillators.
These and other objects and features of the invention are attained by a first aspect of the present invention which includes a phase-locked loop circuit having a plurality of phase-locked loops. Each phase-locked loop has a voltage controlled oscillator operating at a different frequency, the frequency of operation of each voltage controlled oscillator being determined by a divide ratio of the voltage controlled oscillator output frequency to a phase frequency detector. The divide ratio for each phase-locked loop is programmable. A processor circuit coupled to the voltage controlled oscillators generates a control word for programming each of the voltage controlled oscillators. A first portion of the control word programs a divide ratio for a first voltage controlled oscillator and a second portion of the control word addresses an auxiliary register containing a control word for programming a divide ratio for a second voltage controlled oscillator.
Another aspect of invention includes a cellular telephone, having a phase-locked loop circuit for generating a first local oscillator frequency for a first mixer and a second local oscillator frequency different from the first local oscillator frequency for a second mixer. The frequency of the first and the second local oscillators is changed between first and second modes of operation of the cellular telephone. A first phase-locked loop generates the first local oscillator frequency. The frequency of operation of the first phase-locked loop being under programmable control. A second phase-locked loop generates the second local oscillator frequency the frequency of operation being under programmable control. A processor generates a control word coupled to the first and the second phase-locked loops. The control word has bits for setting the operating frequency of the first phase-locked loop and having fewer than the number of bits required to set the operating frequency of the second phase-locked loop.
A further aspect of the invention comprises a cellular telephone includes a transmit circuit, a receive circuit, and first and second mixers in said transmit circuit and third and fourth mixers in said receive circuit. A phase-locked loop circuit has a first phase-locked loop for generating a local oscillator frequency for said first and third mixers, said first local oscillator frequency being different during transmit and receive operation of said cellular telephone. A second phase-locked loop generates a local oscillator frequency for said second and fourth mixer, said second local oscillator frequency being different during transmit and receive operation of said cellular telephone and different from said first local oscillator frequency. A processor generates a single control word for setting the frequency of said first and said, second phase-locked loops, the control word has fewer bits than necessary to control the operating frequency of said first and said second phase-locked loops.
Yet another aspect of the invention includes a method for operating a cellular telephone having a plurality of phase-locked loops. Each phase-locked loop has a VCO operating at a different frequency. The frequency of operation of each VCO is determined by a divide ratio of the VCO output frequency to a phase frequency detector, the divide ratio being programmed by processor. The method includes serially transmitting a control word from the processor to a phase-locked loop circuit for programming a first and a second divide ratio for a first and second VCO, respectively. The control word is decoded to generate a divide ratio for the first VCO and an address of an auxiliary register. The auxiliary register is addressed to read out a divide ratio for the second